Figure 2.

Schematic representation of PDT mechanism of action. The photosensitizer (PS, a photo-activatable molecule) is excited from the ground state to the singlet excited state (¹PS) with light of a specific wavelength. From this excited state, the PS undergoes intersystem crossing to an electronically different excited state lower in energy such as the triplet state (³PS). In its long-lived triplet state the PS reacts with local microenvironment to generate reactive molecular species or free radicals. These reactive species induce cell death. For example, energy from the PS triplet state is transferred to the ground-state triplet oxygen molecules (³O₂) to generate reactive singlet oxygen (¹O₂) molecules.